Abstract

A recent report in BMC Cell Biology examines how the balance of extracellular forces and intracellular contractions regulate
the shape changes required for oligodendrocyte myelination. A failure of remyelination
such as seen in multiple sclerosis could be caused by loss of this balance.

The interplay between intracellular and extracellular forces

During development, all cells undergo enormous changes in cell shape. After a cell
is 'born', it migrates to its final destination, where it then changes its shape to
assume its final role. Very often, this involves the formation of cellular processes,
many of which have specific shapes and functions that are characteristic to the individual
cell types. This process outgrowth and other changes in morphology are supported internally
by a sturdy network of specialized structural proteins that form the cytoskeleton.
In addition, the surrounding extracellular environment, the extra-cellular matrix
(ECM), mediates the changes in cell shape through its mechanical properties. The role
of the ECM becomes particularly apparent when adherent cells (cells that are part
of a tissue) are compared with non-adherent cells (cells that are floating freely
within a liquid, such as blood). Although most adherent cells have a very particular
shape, non-adherent cell types are usually rounded but change shape when they attach
to surrounding tissue [1], suggesting that adherent cells can sense and respond to mechanical signals from
the ECM.

These mechanosensory properties are mediated by special adhesion sites, where the
ECM binds to a family of receptor proteins within the cell membrane. The most significant
of these are the integrins. Binding of ECM ligands to integrins, with the associated
varying degrees of mechanical strain or stretch, promotes the recruitment and linkage
of part of the cytoskeleton, the actomyosin network, to intracellular integrin domains
and thus anchors this network at the lipid membrane [2,3]. The actomyosin network consists of two major components, actin filaments and myosin
molecules, which can slide along each other, creating an intracellular contractile
force (Figure 1a). The relationship between this intracellular force and the strength of cell adhesion
(the extracellular force) could then simply regulate shape such that stronger external
forces would pull the cellular membrane outwards, whereas stronger internal forces
would maintain a rounded shape.

However, cellular events seem to be more complex than this. First, recent findings
have emphasized the importance of functional actomyosin contractile mechanisms for
the regulation of a wide range of cell properties, including tissue formation, cell
migration and cell differentiation [4]. Second, in contradiction to this simple model, low contractile forces generally
yield membrane-rich, bulgy cell types, whereas strong contractions lead to the formation
of highly structured cell shapes. Finally, different cell types reportedly have different
ECM rigidity preferences for the induction of their particular shape. All this suggests
that cellular shapes are determined by a precisely regulated interplay of intracellular
contractile forces and extra-cellular attachment.

Interplay of forces in myelination

A particularly striking example of this interplay is the neural cell lineage, which
gives rise to neurons, astrocytes and oligodendrocytes in the central nervous system
(CNS). Developmentally, all three neural cell types develop from the same multipotent
stem cells. However, neurons, which are generated first, prefer relatively soft surfaces
for elaboration and branching of axons and dendrites. These softer substrates possibly
correspond to the environmental conditions at the time of initial pathfinding of neuronal
processes. In contrast, in recently published work in BMC Cell Biology [5], Simons and colleagues show that the myelin-forming oligodendrocytes that develop
later form their highly processed morphology and extensive myelin sheets best on more
rigid surfaces.

This seems logical if we take a closer look at the developmental context of the formation
of the insulating myelin sheath around axons. Once the migratory oligodendrocyte precursor
cells (OPCs) have reached their destination and start to establish contact with an
axon, their processes change from exploring the unstructured extracellular environment
of the presumptive myelinated tract, as required for migration, to the establishment
of close contact with the highly structured (and therefore probably more rigid) surface
of the axon, initiating the process of wrapping it with membranous sheets that will
eventually become the compact myelin sheath [6]. An increase in intracellular force would therefore be necessary to enable the opposing
forces to be matched and promote the next stage of oligodendrocyte development – the
elaborate shape changes that accompany myelination (Figure 1b).

The findings of Simons and co-workers [5] also provide information about these intracellular mechanisms. Investigation of the
role of intracellular contractility in differentiation and myelination identified
myosin IIB, one of the major components of the actomyosin cytoskeleton, as a central
player in generating intracellular force. In cell culture experiments, softer surfaces
inhibit process outgrowth, as would be predicted if oligodendrocyte differentiation
is normally associated with increased levels of intracellular force to match the increased
rigidity of the axons. This effect can be overcome by pharmacological inhibition of
myosin IIB, which will reduce intracellular contractions and thus better match intracellular
force with the lower extracellular attachment efficacy provided by less rigid substrate
(Figure 1b).

These findings are of particular interest for two reasons. The first is that they
offer a clue as to how one might explain the rather surprising reported effects of
myosin IIB inhibition on myelination in culture. Wang et al. [7] showed that myosin IIB inhibition in a neuron-oligo-dendrocyte co-culture system
significantly enhanced the formation of the myelin sheath, a change that resulted
from individual oligodendrocytes forming more wrapping processes than cells in untreated
control cultures. In complete contrast, inhibition of myosin IIB in co-cultures of
Schwann cells (the myelinating cells of the peripheral nervous system) with neurons
inhibited myelination, and cellular morphology was characterized by aberrant process
outgrowth. In short, while Schwann cells react as would be predicted, oligodendrocytes
exhibit a behavior that contradicts the conclusions obtained from previous experiments.
This might reflect important differences in the biology of the Schwann cell and the
oligodendrocyte, in particular in respect to their adjustment to in vitro conditions: the extracellular forces on the Schwann cell appear to be similar in culture
and in vivo, whereas the extracellular forces on oligodendrocytes in culture are potentially
weaker than in vivo. The presence of a basal lamina on the non-axonal side of the Schwann cell but not
the oligodendrocyte both in vitro and in vivo might be one means of retaining such an extracellular force.

Interplay of forces in multiple sclerosis

The second, and more important, reason for interest in the findings of Simons and
colleagues [5] is that they offer explanations as to why remyelination might fail in the demyelinating
disease multiple sclerosis (MS) [8]. In MS, unknown molecular triggers induce an inflammatory reaction in the brain leading
to an invasion and activation of immune cells (B and T lymphocytes and macrophages)
and/or the produc tion of antibodies directed against myelin components. These events
lead to the damage and degeneration of the myelin sheath. Remyelination does occur
in the early stages of the disease as intrinsic mechanisms mediate the recruitment
of OPCs, which then align with the denuded axon and regenerate the sheath. However,
this repair mechanism eventually fails, for as-yet unknown reasons. An implication
of the results of Simons and colleagues [5] is that increased rigidity in the scarred brain may play a role by unbalancing the
intracellular and extracellular forces and inhibiting oligodendrocyte differentiation
(Figure 1b).

How might the rigidity of the chronically demyelinated CNS be altered? Astrocytes,
the third cell type derived from the neural lineage, provide nutrients to neurons
and oligodendrocytes, give biochemical support to the cells forming the blood-brain
barrier and, in particular, mediate the repair and scarring processes in the CNS following
traumatic injuries. They respond to pathological insults, including inflammation and
demyelination, with so-called reactive gliosis. On a cellular level, this is characterized
by an upregulation of intermediate filament proteins, leading to the formation of
a prominent intermediate filament network directly underneath the plasma membrane,
rendering the cellular texture more fibrous [9]. Furthermore, pronounced changes in expression of adhesion molecule genes have been
described, which would result in an altered ECM composition compared with that of
initial myelination. In demyelinated plaques, reactive astrocytes are the most abundant
cellular component, and astroglial scars have been described as being more rigid than
their surrounding tissue. These properties might alter force-sensing integrin function
in the oligodendrocyte, unbalancing the cellular forces and inhibiting remyelination.

The main implications of the findings of Simons and colleagues [5] are, therefore, that a particular balance of extracellular adhesion, matrix rigidity
and intracellular contractile forces mediated by the oligodendrocyte actomyosin cytoskeleton
is required for successful myelination and remyelination. One interesting prediction
implied by these data is that extracellular cues that do not in themselves alter rigidity,
but that do change the activity of signaling molecules regulating intracellular force,
could also inhibit remyelination. As discussed above, the predominant pathway involved
in the signaling mecha nisms underlying mechanosensing and mechanotransduction is
binding of ECM ligands to integrin receptors in the membrane. The activation of integrins
by mechanical forces results in the recruitment of intracellular mediators that signal
through a pathway involving RhoA and its downstream effector ROCK to activate force-generating
myosin II (Figure 1a). The observation that the inhibitory effects of myelin debris on OPC differentiation,
myelination and remyelination are mediated by RhoA-ROCK signaling [10] is consistent with this hypothesis [5]. Subsequent pharmacological disruption of the ROCK pathway, inhibiting myosin IIB
and thus actomyosin contractility, was able to enhance oligodendrocyte differentiation
[10]. Clearly, the signaling molecules that regulate intracellular force now provide an
intriguing source of candidates for drug discovery programs aimed at enhancing remyelination
(Figure 1b).